When heat generated within a portable information device such as a notebook type computer of recent years is transferred to a surface of a device enclosure, and raises temperature of the surface of the device enclosure, the heat on a portion of the enclosure surface that stays in contact for a long time with a body of a user of the device gives an uncomfortable feeling to the user. Sources of the heat within the computer are mainly a CPU and power supply, and a surface temperature of the CPU, in particular, reaches such a temperature that exceeds approx. 100° C.
As the latest techniques under these circumstances, there have been proposed some techniques in that thermal insulators are used to block heat between a heating component in the device and the device enclosure.
There has been proposed, for instance, a notebook type computer of a structure including a thermal insulator for isolating between a heating component in the device and a device enclosure, a heat sink disposed to a back surface of a display unit, a heat pipe for conducting heat generated in the device to the heat sink, and an air vent, as disclosed in Japanese Patent Laid-open Publication, No.11-202978. Temperature rise on the enclosure surface can be reduced to some extent by using the technique taught in the publication, 11-202978.
However, it is necessary to increase a thickness of the thermal insulator in order to attain an effect of reducing an amount of heat transferred to the surface of the device enclosure, because the effect is small if thermal insulation property of the thermal insulator is low. On the other hand, since low-profiling and weight-reduction are desired in these days for notebook type computers, it is also necessary for the thermal insulators to be small and light weight.
Besides, the heat generated within a device may cause an adverse effect to external expansion terminals such as a random-access memory (RAM) card and a local area network (LAN) card, and leads them into malfunction.
As ordinary thermal insulators, fibrous body such as glass wool, and foam body such as polyurethane foam and the like are used. In order to improve insulating properties of these thermal insulators, however, it is necessary to increase their thicknesses. The thicknesses are therefore not suitable to a need to save or use spaces efficiently due to limitation in the spaces available for installation of the thermal insulators.
As one of the ways to solve the problem, a vacuum thermal insulator including a core member, which maintains a space, and an enveloping member, which shields the space from the open air is available. Powdery material, fibrous material, continuously-formed foam body, and the like are used generally as the core member. However, even more efficient vacuum thermal insulators are now in demand.
Thus, Japanese Patent Laid-open Publication, No.60-33479 teaches a vacuum thermal insulator characterized by uniformly-distributed powdery carbon into pearlite powder for improving efficiency of the core member. It also discloses the vacuum thermal insulator including the powdery carbon consisting of carbon black. The thermal insulation property can be improved by 20% under the optimum condition by the uniformly distributing carbon black into the pearlite.
Furthermore, Japanese Patent Laid-open Publication, No.61-36595 teaches a vacuum thermal insulator characterized by uniformly-distributed carbon powder in various powdery pulverulent substances. In one of its embodied examples, carbon black is distributed uniformly into silica having 100 nm in particulate diameter, and obtains an improvement of 20% in the thermal insulation property under the optimum condition.
Moreover, Japanese Patent Examined Publication, No.08-20032 discloses a vacuum thermal insulator which uses fine powder produced from fumes generated in the production of ferrosilicon. It also discloses the vacuum thermal insulator in which the fine powder contains at least carbon in an amount of 1 wt % or more. This thermal insulator exhibits an improvement of 23% in the thermal insulation property.
With regard to the pearlite in Patent Publication 60-33479, the silica having particulate diameter of 100 nm in Patent Publication 61-36595, and the fumes generated in the production of ferrosilicon in Patent Publication 8-20032, all containing powdery carbon or other forms of carbon, however, the pearlite, the silica having particulate diameter of 100 nm, and the fumes generated in the production of ferrosilicon, as used for the base material do not exhibit any distinguishable effect as the core member of the vacuum thermal insulator. Therefore, they do not improve the thermal insulation property substantially, and their effects of improvement are only up to about 20% when compared with other vacuum thermal insulators, even though these materials are intended for a large improvement by containing powdery carbon and the other forms of carbon.
In the specification that uses carbon black as the powdery carbon, the carbon black exhales gas over time, since it generally is a sooty product obtained through imperfect combustion of oil content and it contains organic gas as impurity. Thus, it has had a problem that an increase in an internal pressure of the vacuum thermal insulator causes degradation of the thermal insulation property. In addition, reaction-active radicals such as carbonyl group present in the carbon black at ends of their molecular structures react with moisture in air so as to also produce gas over time, thereby increasing the internal pressure of the vacuum thermal insulator and degrading the thermal insulation property in the similar manner.
Porous bodies are generally used as the core member, which can be classified broadly into one of continuous foam, fibrous group, and pulverulent group.
Among them, silica powder is often used as one of the pulverulent group vacuum insulator materials. Vacuum thermal insulator that uses silica powder is superior in the thermal insulation property over a long duration of time, although it falls behind the fibrous group material in the initial thermal insulation property.
However, because being powdery, this material has poor workability, and is hardly formed in an irregular shape since the powder is to be enveloped in an inner enveloping member when being used. Moreover, it also impairs work environment as the powder disperses when being disposed. There have been some attempts for improvement by making silica powder into compact form. However, various kinds of binder need to be used because of difficulties in molding the silica powder itself into porous bodies.
For instance, Japanese Patent Examined Publication, No.04-46348 discloses a vacuum thermal insulator having a formed body made of wet silica mixed and compressed with fiber reinforcement.
This includes a form having radiation inhibitor compressed, if there are large gradients in temperature between the wet silica and the fiber reinforcement and between walls in which the vacuum thermal insulator is used.
Moreover, Japanese Patent Examined Publication, No.05-66341 —discloses a vacuum thermal insulator including a formed body made of dry silica, wet silica and fiber reinforcement, which are mixed, distributed, and compressed.
A formed body is formed in which the dry silica and the wet silica supplement each other by their respective features of low coefficient of thermal conductivity and easiness of press-working, with addition of mixing fiber reinforcement into it.
However, it is not feasible to form the body only with the silica powder.
Even if the wet silica is mixed, stirred, and compression-formed with fibrous material, as taught in Patent Publication, No. 04-46348, the formed body is so brittle that it crumbles easily when being picked up with a hand. In addition, it also raises heavy powdery dust thus deteriorating workability and ease of handling. If it is formed into a cylindrical shape, for example, it crumbles so quickly. Also, since lacking flexibility, the body has a limitation in the field of applications.
Furthermore, even if being formed by mixing and stirring the wet silica, dry silica and the fiber reinforcement, and compression forming them, as taught by Patent Publication, No.05-66341, the body is not easily formed, and is brittle because it contains the wet silica mixed to it. In addition, it raises heavy powdery dust, and lacks flexibility.